1. Field of the Disclosure
The present invention relates to an antenna arrangement and to a beam forming device, e.g. for use in an active imaging device for imaging a scene.
2. Description of Related Art
Active imaging systems are becoming more and more popular at ultra-sonic, microwave, millimeter and terahertz frequencies for a number of applications including medical and security applications.
The arrangement of transmitter (herein also called “illumination unit” or “transmit unit”) and receiver (herein also called “receive unit”) in an active imaging system may take on many different forms. In an embodiment relevant for the present invention multiple transmitters and receivers work together to form perform beam forming, e.g. in a MIMO radar or a MIMO active imaging system. There are predominately two different types of MIMO radars. The first type is called statistical MIMO, in which the antennas (generally the “transmit antennas” and the “receive antennas”) are placed far apart from each other to provide different views of the object (generally the “scene”). The second type of MIMO is called beam forming (or co-located) MIMO in which the antennas are placed close to each other and form a sparse array. They act together to form a “virtual” beam forming array or “virtual phase centers”. MIMO beam forming can be used in one dimension (1D MIMO) or in two dimensions (2D MIMO). The present invention can be used for both of these cases.
For MIMO beam forming, the combination of the transmitting and receiving antennas form a set of virtual phase centers which are located in space. Each phase center is obtained by convolution of the phase centers of the transmit and receive antennas. For an optimum radiation pattern for the final resulting beam, the virtual phase centers need to be separated with an equidistant linear spacing (ideally with spacings smaller than the wavelength/2). To achieve this in practice is very challenging, since the transmitter antennas need to be placed very close to the receiver antennas to maintain the linear spacing of the virtual phase centers.
J. H. G. Ender, J. Klare, “System Architectures and Algorithms for Radar Imaging by MIMO-SAR”, IEEE Radar Conference 2009 describes a 1D MIMO beam forming arrangement in which the transmitter antenna blocks (Tx blocks) are optimally placed on the outside and the receiver antennas (Rx antennas) are placed in the middle of the antenna arrangement. Such an arrangement is regarded as optimum since the total physical size of the antenna is only slightly larger than the resulting aperture size set by the location of virtual phase centers. In this paper the basic spacing rules are described to achieve a linear spacing of the virtual phase centers. A similar arrangement is disclosed in J. Klare, O Saalmann, H. Wilden, “First Experimental Results with the imaging MIMO Radar MIRA-CLE X”, EUSAR Conference 2010.
S. Ahmed et al, “Near Field mm-Wave Imaging with Multistatic Sparse 2D Arrays”, Proceedings of the 6th European Radar Conference 2009, p. 180-183 describes three different 2D MIMO possibilities. In all cases the spacing of the closest Tx block to Rx block is maintained as (Tx to Tx antenna spacing)/2 in both dimensions.
X. Zhuge, “Short Range Ultra-Wideband Imaging with Multiple-Input Multiple Output”, PhD Thesis, Delft University of Technology 2010 describes in chapter 4 many different 2D MIMO arrangements. On page 101 the authors conclude that the uniform 2D rectangular arrangement of the transmitter and receiver antennas (as seen on page 90, FIG. 4.3) has the largest effective aperture. This means that this 2D arrangement has the largest aperture size for a given physical size of antennas. This uniform 2D rectangular arrangement of the transmitter and receiver antennas is the same as proposed S. Ahmed (see above). As already stated for the virtual phase centers to be linearly spaced for such an arrangement the Tx block to Rx block spacing has to have a spacing corresponding to (Tx to Tx antenna spacing)/2.
V. Krozer et al, “Terahertz Imaging Systems with Aperture Synthesis Techniques”, IEEE Transactions on Microwave Theory and Techniques, Vol. 58, No. 7, July 2010, pp. 2027-2039 describes a variety of different systems. Section IV.C (page 2033) describes in particular a 2D MIMO array in FIG. 7 in which the Rx antennas are placed close together in the middle and the Tx antenna are widely spaced on the outside. This is one way to implement a 2D MIMO array with the required Tx to Rx block spacing. However such a solution has a number of drawbacks which include that the aperture is of low efficiency (array is physically large with a correspondingly small virtual aperture size), that the receivers are placed close together causing coupling problems and that certain beam angles use a reduced number of receiver causing reduction in resolution.
However, none of these documents provides a solution how this required Tx block to Rx block spacing can be achieved in practice.
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventor(s), to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.